Organic farmers use a wide variety of tools and strategies to control weeds withoutsynthetic chemicals. Successful organic farmers continually adapt their weedmanagement practices as weed populations shift. Producers should have a goodunderstanding of the philosophies and legalities of organic farming before they plantheir weed management strategies. A brief overview of organic agriculture follows;for further details, see Iowa State University Extension publication

A weed is simply a plant that takes advantage of unused resources made available by croppingpractices. Nutrient availability is usually high at thebeginning of the season and decreases during theseason. Water availability depends on the weatherpattern of the particular year, but it is typicallyhigher in the beginning and lower at the end of theseason. Light under the crop canopy that isavailable for weeds is high at beginning, low in themiddle, and generally high again at the end of theseason when crop leaves are senescing. Knowingthe ecology (germination and growth period) of the weeds on your farm is key to organic weedmanagement.

Why Organic?

Environmental, economic,and food safety concernsare among the many rea-sons why some farmerschoose organic production.For example, certifiedorganic soybeans in Iowaaveraged a 200–300 per-cent premium price overconventional soybeans. Onthe environmental front,organic farmers hope toreduce the 240 millionpounds of herbicidesapplied each year in theMidwest. Organic farmersalso express concernsabout weed resistance to herbicides and thetransfer of herbicide-resistant genes to wild plantsthat may occur with increasing reliance onherbicide-resistant crops.According to the federal Organic FarmingProduction Act (OFPA) of 1990, and the USDANational Organic Program (NOP), anyone sellingproducts as “organic” must follow a set of pre-scribed practices that includes avoiding syntheticchemicals in crop and livestock production and inthe manufacturing of processed products. Organiccertification agencies, serving as the required thirdparty certification, include private agencies, such asthe Organic Crop Improvement Association (OCIA),and public programs, such as the State of IowaDepartment of Agriculture and Land Stewardship(IDALS) organic certification program. For moreinformation, see

Organic crops are required to be grown in rotations,asdemonstrated by the corn-soybean-oat-alfalfa rotation,shown at the ISU Neely-Kinyon Farm.

under any remaining rye plant residues to avoidcompetition with the germinating crop.There is a strong correlation between biomass,tillering (multiple stems), and weed competitive-ness. Barley, for example, has a more extensivetillering system and is more competitive withweeds than wheat. Because small grains also areplanted in narrower rows, these crops are morecompetitive with weeds than corn or soybeans.Many organic farmers opt to fallow a field to a ryefor an entire season if weeds have presented apersistent problem in the past.Maintaining soil fertility through crop rotations,cover crops, intercrops, and biologically-basedfertilizers will enhance the competitiveness of thecrop plant and inhibit weed growth. Reportsindicate that humic and fulvic acids in compost maymitigate weed seed germination. Small-seededweeds also may be more susceptible to pathogensassociated with high organic matter in compost.Compost placed close to the crop plant reduces theamount of nutrients available to weeds betweencrop rows. Mulch also is effective in suppressingweed establishment.

Crop rotations are thefoundation of organicfarming. Organic certifica-tion requires that a smallgrain and/or legume beplanted after row crops tomaintain soil health andbiologically based pest management. As anexample, if the legume is plowed under as a covercrop in the fifth year, four years of row crops couldbe grown prior to the green manure crop year. Thesame crop cannot be grown in sequential years;thus, soybeans cannot be grown in the same fieldyear after year. The ideal crop preceding soybeansis winter rye. Soybean fields are rotated to a smallgrain (oats, barley, wheat, or rye) or corn.A typical crop rotation in Iowa is corn followedby a winter cover of rye, soybeans, and oats with anunderseeding of alfalfa or red clover in the thirdyear. Rye, with its allelopathic properties, will helpprevent weed establishment. In the spring, rye thatis less than 8" in height can be killed with a fieldcultivator. Taller rye plants should be mowed or cutwith a stalk chopper before cultivating. A secondcultivation or disking may be necessary to turn